Many heavy vehicles, such as tractor-trailer combinations, use air braking systems. Air under pressure is selectively directed to the vehicle brakes, including the trailer brakes, to slow and stop the vehicle. The air pressure is provided by a compressor and one or more reservoirs on the tractor. Supply air and service air (control pressure air) are directed over two lines from the tractor to the trailer. The control pressure air controls operation of the service relay valve on the trailer, which in turn directs supply air to the trailer brakes as needed.
The service control pressure for the trailer brakes is normally set by the vehicle brake pedal, which also simultaneously controls the tractor brakes. In addition, many vehicles have a manually operated trailer control brake valve, also known as a trailer control valve, that is operable to selectively apply the brakes of only the trailer. The trailer control valve may be a hand operated push-type valve, or may be a hand operated valve responsive to rotary force, for example, turning a handle. This latter type of valve is exemplified by the TC-7™ trailer control brake valve of Bendix Commercial Vehicle Systems LLC. This particular valve has a ramped cam and cam follower that engage each other to translate rotary motion of the handle into linear motion of a valve element, thereby to control, in a graduated manner, service control pressure to the trailer brakes.